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Organizing Pneumonia (OP)

  • What is it?
    • OP is a form of chronic inflammatory lung disease that involves the respiratory bronchioles, alveolar ducts, and alveoli.
  • Classified as:
    • A type of interstitial lung disease (ILD).
  • Caused by:
    • Secondary OP: Associated with a known cause such as
      • infection,
      • drugs,
      • radiation, or
      • systemic diseases.
    • Cryptogenic OP (COP): Idiopathic with no identifiable cause.
  • Resulting in:
    • Impaired gas exchange and restrictive lung disease.
  • Characterized by:
    • Patchy alveolar and bronchiolar inflammation.
    • Formation of fibroblastic plugs (Masson bodies).
    • Potential progression to fibrosis.
  • Anatomically affecting:
    • Respiratory bronchioles, alveolar ducts, and alveoli.
  • Pathophysiology:
    • Initial injury occurs at the level of the respiratory bronchioles and alveolar ducts.
    • Inflammatory response leads to granulation tissue formation within these structures and extends into alveoli.
    • Failure of resolution results in organizing fibrosis.
  • Diagnosis:
    • Clinical:
      • Symptoms include dyspnea, cough, and sometimes fever or weight loss.
      • Lack of response to antibiotics for suspected pneumonia.
    • Imaging:
      • Radiologic Principles:
        • Consolidation and ground-glass opacities.
        • Perilobular patterns and nodules.
      • Applied Anatomy:
        • Parts: Respiratory bronchioles, alveolar ducts, alveoli.
        • Size: Lesions often span multiple secondary pulmonary lobules.
        • Shape: Rounded, nodular, linear, or perilobular.
        • Position: Can involve any lung region without lobar preference.
        • Character:
          • Ground-glass opacities
          • Reticulations
          • Consolidation,
          • Atoll sign (reverse halo sign):
            • Central ground-glass opacity surrounded by a
            • ring of consolidation,
            • historically considered characteristic but not pathognomonic.
          • Nodules
          • Masses
        • Time: Subacute onset over weeks to months.
      • CXR:
        • Patchy areas of consolidation.
        • Peripheral and lower lobe predominance.
      • CT:
        • Key Findings:
          • Consolidation with ground-glass opacity.
          • Perilobular pattern (arcade-like sign).
          • Atoll sign (reverse halo sign).
          • Small nodules or masses.
          • Absence of honeycombing.
      • PET-CT:
        • Hypermetabolic activity in inflamed or fibrotic regions.
        • Useful to differentiate OP from neoplastic lesions.
      • Other relevant modalities:
        • MRI has limited utility but may show high signal intensity on T2-weighted images.
    • Laboratory findings:
      • PFTs: Restrictive pattern with reduced diffusion capacity (DLCO).
      • Bloods: Elevated inflammatory markers (e.g., ESR, CRP).
      • Pathology: Histology reveals intra-alveolar granulation tissue and organizing fibrosis.
  • Treatment:
    • First-line: Corticosteroids (e.g., prednisone).
    • Address underlying cause if secondary OP.
    • Antibiotics for concurrent infection.
  • Prognosis:
    • Generally favorable with appropriate treatment.
    • Relapses can occur, requiring long-term monitoring.
  • Radiological Implications:
    • Follow-up imaging to monitor treatment response.
    • Recognition of patterns (e.g., Atoll sign) is crucial for differentiation from other diseases.
  • Key Points and Pearls:
    • OP can mimic infectious pneumonia but does not improve with antibiotics.
    • Diverse findings: COP can appear as patchy consolidations, nodules, ground-glass opacities, or combinations thereof.
      • Atoll sign (reverse halo sign): Once thought to be pathognomonic, it is now known to occur in other conditions (e.g., invasive fungal infections, vasculitis).
    • Can present as focal masses, raising suspicion for malignancy.
    • Multidisciplinary approach often required for diagnosis and management.